[airtunes ] Unify airtunes and use libshairplay by default (as on windows)

This PR allows all platforms which use the unified depends to use libshairplay for our airtunes implementation. This lib is already used on windows and has an active upstream dev (not the case for the current libshairport).

I have tested it for osx and ios - i was not able to compile win32 (still ffmpeg stuff i guess?) but i packaged the dll and uploaded it to the mirrors. @wsoltys could you give it a shot - not sure if i did everything right (or we could fire off a jenkins build to verify maybe?).

@wsnipex could you try and package libshairplay from here for our ppa? (this is optional though - we can still work with libshairport).

https://github.com/juhovh/shairplay/commit/139d5ef55564514c31f02dd82cef91236c9ff523

Beside that our configure will default to libshairplay and if not found fallback to old libshairport. (didn't want to screw up rbpi and other linux distros). @gimli maybe you could figure out what needs to be done for the rbpi depends for letting it built with libshairplay?

I know that the real goal is to move all that stuff out to an addon - but tbh i fear python atm and so i try to unify core for now.

Benefits of this - we have working password protection for music streams and track changing seems to be instant (libshairport needed 3 secs for changing track). We loose nothing as far as i have tested.

I aim at the may merge window...